The apparatus to be disclosed involves a novel, useful, and non-obvious burner apparatus for a process heating system, a novel, useful, and non-obvious process heating system based on this burner apparatus, and a novel, useful, and non-obvious method to provide a process heat stream for a heat-consuming process based on this technology.
Biomass fired process heaters have been used for several decades to improve the economies of industrial drying processes, particularly for drying of biomass products. Biomass fired process heaters have allowed expensive fossil-fuels, such as natural gas, liquefied petroleum gas, and fuel oil, to be replaced with biomass fuels that are often available as a byproduct—or even as a direct product—of the industrial process. Several previous inventions have been used to that end.
The most popular biomass-based process heating system in current use is based on a cyclonic suspension burner. A cyclonic suspension burner uses biomass that has been ground to a powder. Often this biomass is obtainable directly from the product stream at a point downstream from the drying process. The ground biomass is injected into a refractory-lined vertical cylinder in which a suspended vortex is created by injecting ambient air tangentially into a cylindrical combustion chamber. The ground biomass and air mixture are suspended in the vortex. A source of ignition, such as a fossil fuel burner, is provided to initiate the combustion process, which after a short time becomes self-sustaining. Hot combustion products and unconsumed secondary air are sent to a drying system, such as a rotary drum dryer, as the process heat source. This process heat source is used in the rotary drum dryer to dry wet biomass feedstock. As the biomass feedstock is dried, a part of the final product is removed, grinded, and sent back upstream to the cyclonic suspension burner as the ground biomass fuel source. A largely self-sustaining process is thereby created.
Several disadvantages have been found with process heating systems based on cyclonic suspension burner technology. First, the cyclonic suspension burner generates very high temperatures in the combustion chamber. To prevent damage to combustion chamber components, the chamber must be lined with large quantities of refractory materials. Typically, several tons of refractory materials must be installed into the large burner. The use of these refractory materials increases the cost of the burner and creates significant maintenance issues. Refractory materials tend to have poor mechanical properties, such as brittleness. It is not feasible to install the refractory material at the factory where the burner is manufactured because the refractory material would be damaged during shipping. Therefore, the refractory material installation must be performed at the biomass drying plant. The cost to install these refractory materials makes a low output process heating system—one with less than 10 Mega BTU per hour—economically unfeasible. Further, the refractory materials tend to deteriorate under thermal and physical loading after relative short periods of operation. Typically, after several hundred hours, the refractory materials must either be replaced or substantially repaired. Replacement or repair of the refractory materials requires a complete shutdown of the suspension burner for considerable time, such as several days, before the system can be restarted. Refractory material replacement, repair, and maintenance are expensive due to system downtime and the need for skilled labor.
Second, in a typical biomass process, the wet biomass feedstock must be cleaned prior to drying. During this cleaning process, some of the biomass is removed. This removed biomass cannot be used as end product. Further, the removed biomass cannot be used as fuel for the cyclonic suspension burner since it is not dried and not ground. And, in some cases, the removed biomass would not be suitable for use in the suspension burner even if it was cleaned and dried. Therefore, the removed biomass must simply be disposed to avoid accumulation of unused material at the process site. This removed biomass therefore creates both a disposal cost and lost energy cost that effects the efficiency of the overall production process. Third, cyclonic suspension burner systems tend accumulate significant volumes of ash within the combustion chamber. This ash must be periodically removed to keep the burner operational. To remove the ash, the burner must be completely shut down. Starting, stopping, and re-starting the cyclonic suspension burner consumes considerable time and fossil fuel. Finally, the dried and ground biomass fuel for the suspension burner comes directly from the end products of the process. While this features creates a generally self-sustaining production line, the cyclonic suspension heating system often consumes up to about 20% of the biomass end product.
Other prior art technologies for biomass fueled process heating suffer similar problems as those described above. Typical process heating system heat sources require sorting and cleaning of the biomass fuel to remove less than pristine components, such as tree bark. Systems able to burn less than pure biomass typically required large amount of expensive refractory materials in the combustion chamber. Other systems are mechanically complex and, therefore, prohibitively expensive for small operations that produce less than 10 ton/hour.
There is, therefore, a significant unmet need in the art for a heating source for a process heating system which can use minimally processed solid wood wastes, such as slab-wood, cord wood, or pallet wood, as a fuel source while creating large volume, process heat streams that can be used in rotary industrial dryers to dry biomass products. A system such as this would increase drying process flexibility while reducing operating costs. Further, there is a significant unmet demand for an economical biomass-fired process heating system for small operations where the manufacturing and maintenance costs associated with the use of refractory materials are minimized or avoided altogether.